This invention relates generally to optical reading apparatus, such as a bar code reader, arranged to read coded information by a reading sensor of electronic scanning type.
When reading optically readable information such as a bar code printed on a bar code label, by means of a bar code reader, suitable light is incident on the label to detect reflected light, where the reflected light is transmitted through a lens to image the pattern of the bar code on a sensor which produces an electrical signal indicative of the information from the bar code. However, there are various kinds of recording media carrying optically readable information, such as bar codes. For instance, taking an example of bar code labels, it is to be noted that the reflection coefficient of bar code labels has a variation due to the difference in color, material or due to stain thereon. Since bar code readers detect the information of the bar code from the difference in reflection coefficient between dark (black) bars and light spaces (white bars), the amplitude of the output signal from the reading sensor used in the reader is small when the reflection coefficient is low. On the other hand, when reading a bar code printed on a high reflection coefficient label, the output signal exhibits a large amplitude. In this way, the output signal level changes due to the difference in reflection coefficient of the bar code labels, and therefore, this results in unstable reading because it is difficult to shape the waveform of an electrical signal whose amplitude varies throughout labels.
In addition, since bar code labels are usually illuminated by a light source built in a bar code reader, the intensity of the reflected light changes when the intensity of the incident light changes. Furthermore, when external light is also incident on the label, the intensity of the reflected light is affected even if the intensity of the light from the built-in light source is kept constant. Especially, when a high-intensity external light is applied to the label, the output signal from an amplifier and other circuits following the reading sensor is apt to be saturated resulting in unreadable condition.